The present invention relates to machines for manufacturing paper.
In particular, the present invention relates to twin-wire machines of this type.
Machines of the above type have main and auxiliary endless wires which have portions situated directly next to each other to compact the web therebetween. The wires have parts which converge toward each other to receive the stock or pulp slurry from the headbox, and as the web is compacted between the wires at the portions which move away from the headbox dewatering takes place through one or both of the wires.
Known machines of the above type require the parts of the wires which are directly next to each other to compact the web therebetween to travel along a path which is both straight and curved. Thus, the parts of the wires which are situated directly next to each other to compact the web therebetween are required at one location to assume a given curvature and at another location to be straight. At the curved parts of the wires, the outer wire should travel at a velocity higher than the inner wire because the outer wire has a larger radius of curvature resulting from the thickness of the wires as well as the thickness of the web interposed and compacted between the wires. If the portions of the wires between which the web is compressed respectively travel at different speeds, so that they have the same angular velocities when travelling along a curve, then these wires will necessarily also have different speeds when travelling along a straight path while compacting the web between the wires, with the result that harmful shearing forces and deformation of the web which is formed between the wires takes place. Since the pair of wires must have different speeds one with respect to the other either at the curved part or at the straight part of their joint path of movement, since it is impossible for them to have equal speeds at both of these parts of their joint path, such harmful shearing forces and deformation of the web which is forming cannot be avoided.
In order to at least attempt to solve this problem it has been common practice to select for the wires speeds which will constitute a compromise which will reduce these undesirable effects as much as possible. However, the results have always been far from satisfactory.
A further disadvantage encountered with conventional machines of the above type, particularly those which have comparatively long twin-wire sections, results from the fact that water tends to fall from the auxiliary wire down onto the web carried by the main wire at the location where the auxiliary wire separates itself from the main wire before returning to the joint path of movement. These falling drops of water which become deposited on the web leave permanent and objectionable marks on the finished product.